Diffusion of a vacancy on Fe(1 0 0): A molecular-dynamics study

The diffusion of a surface vacancy on Fe(1 0 0) has been studied at various temperatures by means of molecular-dynamics simulations in conjunction with a many body potential in the context of the embedded atom method. This interatomic potential was recently constructed by fitting its parameters to both experimental and first-principles results. From the analysis of the vacancy jumps, three main diffusion mechanisms have been investigated. The first one corresponds to the migration of the vacancy on the surface layer (intra-layer jumps) by hopping to a neighboring site, while the two others involve the participation of an atom of the second layer (inter-layer jumps) as well. The temperature dependence of the associated diffusion coefficients follows an Arrhenius behavior, from which the migration energies and pre-exponential factors were deduced. It was found that one of the mechanisms corresponding to inter-layer jumps is energetically slightly favored over the two other processes and in addition, its diffusivity is about six times higher than that of the two others. Our results show that the contribution of vacancy diffusion to mass transport is important.     

Energetics of Ge addimers on the Si(1 0 0) and Ge(1 0 0) surfaces: a comparative study

The adsorption energetics of Ge dimers on the (1 0 0) surfaces of Ge and Si has been investigated using the first-principles molecular dynamics method. Four high-symmetry configurations have been considered and fully relaxed. The most stable configuration for Ge dimers on Si(1 0 0) is found to be in the trough between two surface dimer rows, oriented parallel to the substrate Si dimers. These results are consistent with recent experimental studies of the system using the scanning tunneling microscopy (STM), and help to clarify some existing controversies on the interpretation of the STM images. In contrast, for Ge dimers on Ge(1 0 0), the most stable configuration is on top of the substrate dimer row.     

Ordered mixed surface structures formed by coadsorption of dissimilar metal atoms on Cu(0 0 1)

We have found that various ordered mixed surface structures are formed by coadsorption of two dissimilar metal atoms on Cu(0 0 1) at room tepmerature, using low-energy electron diffraction (LEED) I-V analysis. As coadsorbates, we employed Mg, Bi, Li and K, and surface structures formed by the coadsorption systems of (Mg, Li), (Mg, K) and (Mg, Bi) are presented. A tensor LEED analysis provided detailed geometries of the coadsorbates and the substrate surface. It was found that the surface structures in the above three coadsorption sytems exhibit the restructuring of the Cu(0 0 1) surface. The phase separation into individual adsorbates does not take place, implying that some additional stabilization arises. We demonstrate two origins for the stabilization of the ordered mixed surface structures on Cu(0 0 1). Structures and features formed by the individual adsorption of Mg, Bi, Li and K atoms on Cu(0 0 1) are described first, then those of (2√2×√2)R45°-Mg,Li, (√5×√5)R26.7°-Mg,K, c(2×2)-Mg,Bi, and c(6×4)-Mg,Bi structures formed by the coadsorption are presented. We consider on the basis of the determined structural parameters the question why ordered mixed surface structures are formed instead of the phase separation.     

Dissociation pathways of oxygen on copper (1 1 0) surface: a first principles study

We have carried out first principles calculations to examine the adsorption and dissociation of oxygen on the Cu (1 1 0) surface. At low coverage, our results indicate two absorption species: a peroxo- and a superoxo-like species which have direct implications to the dissociation process. In agreement with experimental studies, the most favourable absorption site for O₂ is found to be the 4-fold hollow site. The dissociation barrier at this site is only 150 meV, which is consistent with experimental findings. Static calculations also reveal other possible dissociation pathways with significantly different energy barriers. First principles MD simulations are used to probe the dynamics of the dissociation process. We find only selected pathways are likely to be utilised because of interaction with the surface.     

Molecular dynamics study of cluster impact on the (0 0 1) and (1 1 0) surfaces of fcc metals

Molecular dynamics simulations of the impact deposition of metal clusters on fcc metal surfaces are presented. Two-dimensional elongated islands are formed when the incident cluster travels parallel to the surface. For perpendicular incidence the results of the impact event are very sensitive to the relative cohesive properties of the cluster and substrate atoms.     

TOF-SARS study of hydrogen adsorption and desorption kinetics on Si(1 0 0)

The kinetics of atomic hydrogen isothermal adsorption and desorption on a Si(1 0 0) surface was studied using the time-of-flight scattering and recoiling spectrometry technique at temperatures below and above the thermal desorption threshold. A continuous decrease in saturation coverage with temperature under constant atomic hydrogen exposure has been observed in both regions for temperatures in the range 325-820 K. For T_S=500-650 K, the decrease is described by a kinetic model where Eley-Rideal (ER) abstraction is responsible for hydrogen removal from the surface and hydrogen coverage depends on the temperature due to the changing rate of migration from precursor to primary monohydride sites. For T_S=650 K and higher, in addition to the ER abstraction, the thermal desorption from primary monohydride sites leads to a further decrease of the saturation coverage. The first-order desorption rates after source shut-off have been measured and an activation barrier of 1.89 eV has been obtained.     

Growth and alloying of submonolayer Cu on Pt(12 12 11)

A detailed comparative study of Cu growth on the vicinal surface Pt(12 12 11) has been performed with helium atom scattering (HAS) and medium energy ion scattering (MEIS). These techniques are ideally complementary in that HAS is extremely sensitive to the electronic structure of the surface layer, while MEIS accurately probes the atom core structure of the surface. By combining these two techniques it has been possible to measure the initial structure of Cu on the stepped Pt surface and to study how the Cu redistributes across the surface and into the bulk on annealing. The HAS and MEIS data identify two significant diffusion transitions on the surface occurring at temperatures of 450 and 600 K.     

A single vacancy diffusion near a Fe (1 1 0) surface: A molecular dynamics study

Both the formation energies, the intra-layer and inter-layer diffuse migration energies of a single vacancy in the first nine layers of the Fe (1 1 0) surface have been calculated by means of molecular dynamics in conjunction with the many-body potential of the embedded-atom method (EAM). The results show that the effect of the surface is only down to the fourth layer for the formation energy of a single vacancy. However, the surface presence affects the migration energy down to the seventh layer. It is easier for a vacancy in the first layer to form and to migrate than in the other layers and the bulk. For the inter-layer migration, a vacancy in each of the first six layers is favorable to migrate into the corresponding upper layer, especially for the first three layers.     

A molecular dynamics simulation of TiN film growth on TiN(0 0 1)

The growth of thin TiN films on the TiN(0 0 1) surface during reactive sputtering was simulated by molecular dynamics with the modified embedded-atom method potential. TiN₃ is found to be the smallest epitaxial island and the film grows via the layer mode. Vacancy concentration in the deposited films decreases with increasing the substrate temperature and kinetic energy of incident atoms, resulting from the enhancement of the thermal diffusion and kinetic energy assisted athermal diffusion. To get the stoichiometric TiN film, the N:Ti flux ratio should be larger than unity and be increased with higher incident energy due to the weak adsorption of atomic N on TiN(0 0 1).     

Chain-length and tacticity effects on the conformational behavior of MMA-oligomer thin films on an Au (1 1 1) substrate

Chain-length and tacticity effects on the conformational behavior, including density distributions and conformations, of MMA-oligomer thin films deposited on an Au (1 1 1) substrate at room temperature were investigated using molecular dynamics (MD) simulations. Three kinds of MMA-oligomer thin film with the oligomers containing m repeat units with m = 5, 10, and 20, respectively, were selected to examine the chain-length effects. Three types of MMA-oligomer thin film (made of isotactic, syndiotactic, and atactic PMMA isomers, respectively) were utilized to inspect the tacticity effects. For ease of investigation, the thin film was divided into three regions (the contact, the bulk, and the surface regions) according to the through-thickness distribution of density of the thin film. For short-chain thin films (with m = 5 and 10), the density distribution had a remarkable peak in the contact region of the thin film. The density peak, however, was not evident for the long-chain thin film (with m = 20). Regarding tacticity, the isotactic thin films had a prominent peak density in the contact region as compared with the syndiotactic and atactic thin films. Moreover, the MMA oligomers were found to exhibit a flattened conformation parallel to the Au substrate in the contact region and a slightly flattened conformation in the surface region for both the short-chain and the long-chain thin films. The flattened conformation remained in the bulk region of the long-chain thin film. However, it was not present in the bulk region of the short-chain thin film, which had a randomly orientated conformation–a characteristic similar to bulk MMA. It was also observed that the tacticity of the MMA-oligomer thin films did not have obvious effects on the conformations of the thin films for both the short-chain and the long-chain cases.     

Molecular dynamics description of silver adatom diffusion on Ag(1 0 0) and Ag(1 1 1) surfaces

The self-diffusion processes of single adatoms on Ag(1 0 0) and Ag(1 1 1) surfaces have been studied using molecular-dynamics simulations and a many-body potential derived in the framework of the second-moment approximation to the tight-binding model. Our results for the (1 0 0) surface indicate that, although the migration energy for hopping is lower than that of the exchange mechanism, the exchange diffusion is higher than hopping diffusion for temperatures above 600 K. The migration energy for the hopping mechanism is in very good agreement with the experiment and the results of ab initio calculations. We also find that for the Ag(1 1 1) face the dominant mechanism is the hopping, which exhibit Arrhenius behaviour with two distinct temperature ranges, corresponding to two different migration energies. The diffusion in the high temperature region is mainly due to correlated jumps requiring an activation energy which is in excellent agreement with the experimental data. In addition the temperature dependence of the mean-square-displacements and the relaxations of both surface atoms and adatoms are presented and compared with previous studies.     

Chemisorption of OH on the H-terminated Si(0 0 1) surface

In order to reveal the etching mechanism of the H-terminated Si(0 0 1) surfaces in ultrapure water, first-principles molecular dynamics simulations of H-terminated Si(0 0 1) surfaces interacting with OH molecules were carried out on the basis of density-functional theory. It was confirmed that the interaction between two OH molecules and the surface silicon atom on the step edge breaks the Si–Si back-bond and initiates the etching process.     

A molecular dynamics study on the thickness and post-critical strength of carbon nanotubes

This paper presents a molecular dynamics (MDs) study on the linear, buckling and post-buckling behaviour of carbon nanotubes (CNTs) under pure shortening and pure twisting. Its objectives are (i) to clarify the issue about the most correct thickness value to adopt in the simulation of CNTs using shell models and (ii) to evaluate their post-critical strength. Three CNTs with similar length-to-diameter ratio but different atomic structures (zig-zag, armchair and chiral) are selected for this study. Then, MD simulations are performed to investigate the pre-critical, critical buckling and post-critical behaviour of CNTs under pure shortening and pure twisting. Using available analytical formulae derived from shell models, the influence of CNT thickness on their critical strain and critical angle of twist is investigated. Some conclusions are drawn regarding (i) the most appropriate choice of the thickness value to use in shell models and (ii) the effectiveness of post-critical stiffness and strength of CNTs.     

Electronic structure of bulk and (0 0 1) surface layers of pyrite FeS2

The electronic structure of bulk and (0 0 1) surface layers of iron pyrite FeS₂ have been calculated using a modern ab initio pseudo-potential method. For the bulk pyrite the calculated lattice constant, position parameter of sulfur, and band gap are in agreement with experimental values. For the (0 0 1) surface it is found that surface states form a conducting band. The conducting band and the conducting band tail of the bulk conduction manifold overlap and the width of the band gap is not influenced by the surface state. The surface states arise mainly from the iron 3d orbitals in the topmost layer with a smaller contribution from the sulfur 3p states. The relaxation of iron and sulfur atoms is found to be greatest in the top most layer. The surface energy is calculated to be equal to 1.063 J/m².     

90°coupling in (Fe/Cr/Fe)AFM/Cr/Fe system epitaxially grown on GaAs(0 0 1)

Single-crystalline (Fe/Cr/Fe)_AFM/Cr/Fe structures were epitaxially grown on atomically flat GaAs(0 0 1). Choosing the same thickness of the antiferromagnetically (AFM) coupled Fe layers in the bottom (Fe/Cr/Fe)_AFM structure, their net magnetization is balanced to zero, in particular up to a spin-flop transition when the field is applied along the [1 1 0] direction. For the Cr thicknesses at which the top Fe layer is weakly magnetically coupled to the bottom (Fe/Cr/Fe)_AFM structure, at low fields, the magneto-optical Kerr effect and/or SQUID signal from the sample corresponds to the top Fe layer only. An influence of the Cr spin structure on the magnetization reversal in the Fe layer is reported. In particular, a strong increase of coercivity (by a factor of 12) is found at low temperatures. A 90° coupling is detected which affects the minor loops measured along the [−1 1 0] and [1 0 0] directions.     

Formaldehyde on TiO2 anatase (1 0 1): A DFT study

The adsorptions of formaldehyde molecule on the stoichiometric anatase TiO₂ (1 0 1) surface have been studied by first principles calculations. Four types of adsorption have been investigated at 0.25 ML coverage. Two of them are chemical adsorptions and the other two are physical adsorptions. For the chemical adsorptions, C, O atoms in the formaldehyde molecule form two bonds with the O_2c/O_3c and Ti_5c on the anatase (1 0 1) surface. The CO bond in the formaldehyde molecule is elongated and a dioxymethylene structure forms in the two chemical adsorptions. The OTi_5c interaction can be found in the two physical adsorptions and it is the only contacting point at the interface. No serious internal distortion in the formaldehyde molecule can be found in the physical adsorptions. The LDOS and the difference of the charge density are calculated to investigate the interface bonds of the adsorption. As the adsorption coverage increase, the molecules on the surface repel each other and weaken the adsorptions. For example, the chemical adsorption may become physical adsorption at high coverage.     

Nanometric cutting of copper: A molecular dynamics study

Molecular dynamics (MD) simulations were carried out to study the nanometric cutting of copper. In our approach, the many-body EAM potential was used for the atoms interaction in the copper workpiece. The effect of the tool geometry on the cutting process was investigated. It is observed that with negative rake angle, the chip becomes smaller due to the larger plastic deformation generated in the workpiece. It is shown that as the rake angle changes from −45° to 45°, the machined surface becomes smoother. Besides, both the cutting forces and the ratio of normal force to tangential force decrease considerably with the rake angle changing from negative to positive. In addition, MD simulations with the two-body Morse potential instead of the EAM potential were also carried out to study the effect of different potentials on the simulation results. It is found that there is no big difference in the simulated chip formation and the machined surface under the two different potentials. However, the Morse potential results in about 5–70% higher cutting forces than the EAM potential. It is recommended that the EAM potential should be used for the MD simulations of nanometric machining processes.     

Evolution of interface diffusion and structure of Ni films sputter-deposited on Si(0 0 1)

Ni films were deposited on Si(0 0 1) substrates at 190°C by dc plasma-sputtering in Ar gas in order to investigate the Ni diffusion phenomena into the substrate and its effect on the structure of Ni films. The deposition time was 10, 15 or 30 min. The Ni/Si samples were studied by cross-sectional transmission electron microscopy and Rutherford backscattering. Ni adatoms are confirmed to diffuse into the Si substrate forming a diffusion layer. The diffusion layer consists of two regions. The first one is of homogeneous thickness and locates adjacent to the interface of film and substrate. The second one consists of several isosceles triangles with vertex angle of about 70° standing on the front of first region. Both the thickness of the first region and the number of triangles in the second region increase with increasing deposition time. The compositional ratio of Ni to Si in the diffusion layer is 1.3±0.1. The Ni films have a lot of voids at the interface and a rough surface. The density of the film is estimated to be 25% lower than that of bulk Ni. The thickness of the Ni films does not increase linearly with deposition time.     

First-principles study of the (1 1 0) polar surface of cubic PbTiO3

Under GGA, the cleavage energy, surface energy, surface grand potential, surface relaxation, and surface electronic structure have been calculated for five different terminations of PbTiO₃ (1 1 0) surface by using PAW method implemented in VASP. Taking into account the results of two neutral PbTiO₃ (1 0 0) surfaces, the favorable PbTiO₃ (1 1 0) and (1 0 0) surfaces are the TiO₂-terminated (1 0 0) surface, the PbO-terminated (1 0 0) surface, and the O-terminated (1 1 0) surface successively in view of surface energy minimization. The surface grand potential calculations show that two neutral PbO- and TiO₂-terminated (1 0 0) surfaces are favored in the moderate Pb and O chemical potentials, two mutual complementary TiO- and Pb-(1 1 0) terminations are stable in Pb-poor environment and in O- and Pb-rich conditions, respectively. A non-negligible rumpling of O-terminated (1 1 0) surface is found in the third O₂ layer and large lateral displacements between Ti and O atoms on the PbTiO layer lead to the initial O-Ti-O alignment broken. Different from the Fermi levels of the three nonstoichiometric TiO-, Pb- and O-terminations which are located in the band gap, the Fermi level of the PbTiO- termination is located at the bottom of the conduction band and that of the O₂-termination is located at the top of the valence band due to increment and decrement of the occupation states for polarity compensation.     

A molecular dynamics simulation on surface tension of liquid Ni and Cu

Molecular dynamics simulations of liquid transition metals Ni and Cu have been performed with the tight-binding potential model. The surface tensions of the liquid metals at different temperatures are evaluated using both methods of calculating the work of cohesion and of using the mechanical expression for the surface stress. The calculated surface tension data are compared with available experimental values. The simulated results for Ni are in good agreement with experiment, but those for Cu show about 10–20% underestimation. Comparing with the mechanical method, the data of surface tension calculated using the method of cohesive work show remarkable dependence on temperature, and the estimated temperature coefficients of liquid Ni and Cu are consistent with the experimental data.